Lattice-mismatched semiconductor structures with reduced dislocation defect densities and related methods for device fabrication

ABSTRACT

A method of forming a semiconductor structure includes forming an opening in a dielectric layer, forming a recess in an exposed part of a substrate, and forming a lattice-mismatched crystalline semiconductor material in the recess and opening.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/835,162, filed on Dec. 7, 2017, entitled “Lattice-MismatchedSemiconductor Structures with Reduced Dislocation Defect Densities andRelated Methods for Device Fabrication,” which is a divisional of U.S.patent application Ser. No. 14/977,135, filed on Dec. 21, 2015, now U.S.Pat. No. 9,431,243, issued on Aug. 30, 2016, entitled“Lattice-Mismatched Semiconductor Structures with Reduced DislocationDefect Densities and Related Methods for Device Fabrication,” which is acontinuation of U.S. patent application Ser. No. 14/844,332, filed Sep.3, 2015, now U.S. Pat. No. 9,859,381, issue on Jan. 2, 2018, entitled“Lattice-Mismatched Semiconductor Structures with Reduced DislocationDefect Densities and Related Methods for Device Fabrication,” which is adivisional of U.S. application Ser. No. 12/180,254, filed Jul. 25, 2008,now U.S. Pat. No. 9,153,645, issued on Oct. 6, 2015, entitled“Lattice-Mismatched Semiconductor Structures with Reduced DislocationDefect Densities and Related Methods for Device Fabrication,” which is acontinuation-in-part of U.S. application Ser. No. 11/436,198 filed May17, 2006, which claims priority to and the benefit of U.S. ProvisionalApplication No. 60/681,940 filed May 17, 2005. The entire disclosures ofthese applications are incorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to lattice-mismatched semiconductorheterostructures and, more specifically, to the reduction of dislocationdefects by the formation of a V-groove in a semiconductor substrate.

BACKGROUND

The increasing operating speeds and computing power of microelectronicdevices have recently given rise to the need for an increase in thecomplexity and functionality of the semiconductor structures from whichthat these devices are fabricated. Hetero-integration of dissimilarsemiconductor materials, for example, III-V materials, such as galliumarsenide, gallium nitride, indium aluminum arsenide, and/or germaniumwith silicon or silicon-germanium substrate, is an attractive path toincreasing the functionality and performance of the CMOS platform. Inparticular, heteroepitaxial growth can be used to fabricate many modernsemiconductor devices where lattice-matched substrates are notcommercially available or to potentially achieve monolithic integrationwith silicon microelectronics. Performance and, ultimately, the utilityof devices fabricated using a combination of dissimilar semiconductormaterials, however, depends on the quality of the resulting structure.Specifically, a low level of dislocation defects is important in a widevariety of semiconductor devices and processes, because dislocationdefects partition an otherwise monolithic crystal structure andintroduce unwanted and abrupt changes in electrical and opticalproperties, which, in turn, results in poor material quality and limitedperformance. In addition, the threading dislocation segments can degradephysical properties of the device material and can lead to prematuredevice failure.

As mentioned above, dislocation defects typically arise in efforts toepitaxially grow one kind of crystalline material on a substrate of adifferent kind of material—often referred to as “heterostructure”—due todifferent crystalline lattice sizes of the two materials. This latticemismatch between the starting substrate and subsequent layer(s) createsstress during material deposition that generates dislocation defects inthe semiconductor structure.

Misfit dislocations form at the mismatched interface to relieve themisfit strain. Many misfit dislocations have vertical components, termed“threading segments,” which terminate at the surface. These threadingsegments continue through all semiconductor layers subsequently added tothe heterostructure. In addition, dislocation defects can arise in theepitaxial growth of the same material as the underlying substrate wherethe substrate itself contains dislocations. Some of the dislocationsreplicate as threading dislocations in the epitaxially grown material.Other kinds of dislocation defects include stacking faults, twinboundaries, and anti-phase boundaries. Such dislocations in the activeregions of semiconductor devices, such as diodes, lasers andtransistors, may significantly degrade performance.

To minimize formation of dislocations and associated performance issues,many semiconductor heterostructure devices known in the art have beenlimited to semiconductor layers that have very closely—e.g. within0.1%-lattice-matched crystal structures. In such devices a thin layer isepitaxially grown on a mildly lattice-mismatched substrate. As long asthe thickness of the epitaxial layer is kept below a critical thicknessfor defect formation, the substrate acts as a template for growth of theepitaxial layer, which elastically conforms to the substrate template.While lattice matching and near matching eliminate dislocations in anumber of structures, there are relatively few lattice-matched systemswith large energy band offsets, limiting the design options for newdevices.

Accordingly, there is considerable interest in heterostructure devicesinvolving greater epitaxial layer thickness and greater lattice misfitthan known approaches would allow. For example, it has long beenrecognized that gallium arsenide grown on silicon substrates wouldpermit a variety of new optoelectronic devices marrying the electronicprocessing technology of silicon VLSI circuits with the opticalcomponent technology available in gallium arsenide. See, for example,Choi et at, “Monolithic Integration of Si MOSFET's and GaAs MESFET's”,IEEE Electron Device Letters, Vol. EDL-7, No. 4, April 1986. Highlyadvantageous results of such a combination include high-speed galliumarsenide circuits combined with complex silicon VLSI circuits, andgallium arsenide optoelectronic interface units to replace wireinterconnects between silicon VLSI circuits. Progress has been made inintegrating gallium arsenide and silicon devices. See, for example, Choiet at, “Monolithic Integration of GaAs/AlGaAs Double-HeterostructureLED's and Si MOSFET's” IEEE Electron Device Letters, Vol. EDL-7, No. 9,September 1986; Shichijo et al, “Co-Integration of GaAs MESFET and SiCMOS Circuits”, IEEE Electron Device Letters, Vol. 9, No. 9, September1988. However, despite the widely recognized potential advantages ofsuch combined structures and substantial efforts to develop them, theirpractical utility has been limited by high defect densities in galliumarsenide layers grown on silicon substrates. See, for example, Choi etal, “Monolithic Integration of GaAs/AlGaAs LED and Si Driver Circuit”,IEEE Electron Device Letters, Vol. 9, No. 10, October 1988 (p. 513).Thus, while basic techniques are known for integrating gallium arsenideand silicon devices, there exists a need for producing gallium arsenidelayers having a low density of dislocation defects.

To control dislocation densities in highly-mismatched deposited layers,there are three known techniques: wafer bonding of dissimilar materials,substrate patterning, and composition grading. Bonding of two differentsemiconductors may yield satisfactory material quality. Due to thelimited availability and high cost of large size Ge or III-V wafers,however, the approach may not be practical.

Techniques involving substrate patterning exploit the fact that thethreading dislocations are constrained by geometry, i.e. that adislocation cannot end in a crystal. If the free edge is brought closerto another free edge by patterning the substrate into smaller growthareas, then it is possible to reduce threading dislocation densities. Inthe past, a combination of substrate patterning and epitaxial lateralovergrowth (“ELO”) techniques was demonstrated to greatly reduce defectdensities in gallium nitride device, leading to fabrication of laserdiodes with extended lifetimes. This process substantially eliminatesdefects in ELO regions but highly defective seed windows remain,necessitating repetition of the lithography and epitaxial steps toeliminate all defects. In a similar approach, pendeo-epitaxy eliminatessubstantially all defects in the epitaxial region proximate to thesubstrate but requires one lithography and two epitaxial growth steps.Furthermore, both techniques require the increased lateral growth rateof gallium nitride, which has not been demonstrated in allheteroepitaxial systems. Thus, a general defect-reduction processutilizing a minimum of lithography/epitaxy steps that does not rely onincreased lateral growth rates would be advantageous both to reduceprocess complexity and facilitate applicability to various materialssystems.

Another known technique termed “epitaxial necking” was demonstrated inconnection with fabricating a Ge-on-Si heterostructure by Langdo et at.in “High Quality Ge on Si by Epitaxial Necking,” Applied PhysicsLetters, Vol. 76, No. 25, April 2000. This approach offers processsimplicity by utilizing a combination of selective epitaxial growth anddefect crystallography to force defects to the sidewall of the openingin the patterning mask, without relying on increased lateral growthrates. Specifically, as shown in FIGS. 1A and 1B, in the (111) <110>diamond cubic slip system, misfit dislocations lie along <110>directions in the (100) growth plane while the threading segments riseup on (111) planes in <110> directions. Threading segments in <110>directions on the (111) plane propagate at a 45° angle to the underlyingSi (100) substrate surface. Thus, if the aspect ratio of the holes inthe patterning mask is greater than 1, threading segments will beblocked by the mask sidewall, resulting in low-defect top Ge “nodules”formed directly on Si. One important limitation of epitaxial necking,however, is the size of the area to which it applies. In general, asdiscussed in more detail below, the lateral dimensions (designated as Iin FIG. 1A) in both dimensions have to be relatively small in order forthe dislocations to terminate at sidewalls.

Thus, there is a need in the art for versatile and efficient methods offabricating semiconductor heterostructures that would constraindislocation defects in a variety of lattice-mismatched materialssystems. There is also a need in the art for semiconductor devicesutilizing a combination of integrated lattice-mismatched materials withreduced levels of dislocation defects for improved functionality andperformance.

SUMMARY

Accordingly, embodiments of the present invention provide semiconductorheterostructures with significantly minimized interface defects, andmethods for their fabrication, that overcome the limitations of knowntechniques. In contrast with the prior art approach of minimizingdislocation defects by limiting misfit epitaxial layers to less thantheir critical thicknesses for elastic conformation to the substrate, inits various embodiments, the present invention utilizes greaterthicknesses and limited lateral areas of component semiconductor layersto produce limited-area regions having upper portions substantiallyexhausted of threading dislocations and other dislocation defects suchas stacking faults, twin boundaries, or anti-phase boundaries. As aresult, the invention contemplates fabrication of semiconductor devicesbased on monolithic lattice-mismatched heterostructures long sought inthe art but heretofore impractical due to dislocation defects.

In particular applications, the invention features semiconductorstructures of Ge or III-V devices integrated with a Si substrate, suchas, for example, an optoelectronic device including a gallium arsenidelayer disposed over a silicon wafer, as well as features methods ofproducing semiconductor structures that contemplate integrating Ge orIII-V materials on selected areas on a Si substrate.

In general, in a first aspect, a method of forming a structure beginswith providing a dielectric sidewall, proximate a substrate, with aheight h. The substrate includes a first crystalline semiconductormaterial and a top surface having a first crystal orientation. Thedielectric sidewall defines an opening with a width w exposing a portionof the substrate. A recess, with a maximum depth d and a recessedsurface comprising a second crystal orientation, is defined in theexposed portion of the substrate. A second crystalline semiconductormaterial having a lattice mismatch with the first crystallinesemiconductor material is formed in the recess. The lattice mismatchcreates defects in the second crystalline semiconductor material, andthe defects terminate at a distance H above a deepest point of therecess.

In various embodiments, H may be less than or equal to h+d, d, or w, andthe ratio of h+d to w may be greater than or equal to one. The recessmay have a V-shaped profile. The first crystal orientation may be (100),and the second crystal orientation may different than (100); forexample, the second crystal orientation may be (111).

A third crystalline semiconductor material may be formed above thesecond crystalline semiconductor material, and may be lattice mismatchedto the second crystalline material. The lattice mismatch between thesecond crystalline semiconductor material and the first crystallinesemiconductor material may be less than a lattice mismatch between thethird crystalline semiconductor material and the first crystallinesemiconductor material. A boundary defined by the interface between thesecond and third crystalline semiconductor materials may be proximate aboundary defined by the interface between the exposed portion of thesubstrate and the dielectric sidewall.

The substrate may be removed to expose a bottom portion of the secondcrystalline semiconductor material. The exposed bottom portion of thesecond crystalline semiconductor material may include a non-planarsurface. After removing the substrate, at least part of the bottomportion of the second crystalline semiconductor material may be removed.Removing at least part of the bottom portion of the second crystallinesemiconductor material may include removing a majority of the defects.

A photonic structure may be formed disposed at least partially above thesecond crystalline semiconductor material or disposed at least partiallyinside the opening. Examples of photonic structures may include an LED,a PV cell, and/or a laser diode.

In general, in a second aspect, a semiconductor structure includes asubstrate including a first crystalline semiconductor material. Thesubstrate has a top surface with a first crystal orientation, anddefines a recess with a maximum depth d. The recess includes a recessedsurface with a second crystal orientation, and a dielectric sidewall ofheight h is disposed proximate the recess. A second crystallinesemiconductor material of maximum width w is disposed in the recess, andthe recess defines an interface between the second crystallinesemiconductor material and the substrate. The second crystallinesemiconductor material has a lattice mismatch with the first crystallinesemiconductor material, the lattice mismatch creates defects in thesecond crystalline semiconductor material, and the defects terminate ata distance H above a bottom surface of the recess.

In various embodiments H may be less than or equal to h+d, d, or w, andthe ratio of h+d to w may be greater than or equal to one. The recessmay have a v-shaped profile. The first crystal orientation may be (100),and the second crystal orientation may not be (100); for example, thesecond crystal orientation may be (111).

A third crystalline semiconductor material may be formed above thesecond crystalline semiconductor material, and may be lattice mismatchedto the second crystalline material. The lattice mismatch between thesecond crystalline semiconductor material and the first crystallinesemiconductor material may be less than a lattice mismatch between thethird crystalline semiconductor material and the first crystallinesemiconductor material. A boundary defined by the interface between thesecond and third crystalline semiconductor materials may be proximate aboundary defined by the interface between the exposed portion of thesubstrate and the dielectric sidewall.

A photonic structure may be formed disposed at least partially above thesecond crystalline semiconductor material or disposed at least partiallyinside the opening. Examples of photonic structures may include an LED,a PV cell, and/or a laser diode.

A third crystalline semiconductor material disposed over the secondcrystalline semiconductor material, and a bandgap of the thirdcrystalline semiconductor material may be lower than a bandgap of thesecond crystalline semiconductor material. In addition, the secondcrystalline semiconductor material may be n-doped and the thirdcrystalline semiconductor material may be p-doped. The secondcrystalline semiconductor material may be GaAs and the third crystallinesemiconductor material may be lnP.

In general, in a third aspect, a semiconductor structure includes aphotonic structure and a crystalline semiconductor disposed above thephotonic structure. A surface of the crystalline semiconductor includesa plurality of ridges. A width of one ridge in the plurality of ridgesis less than or equal to a visible light wavelength, and a spacing ofthe plurality of ridges is less than or equal to the visible lightwavelength.

In various embodiments, the photonic structure may be an LED, a PV cell,and/or a laser diode. A metal contact may be disposed above thecrystalline semiconductor, and the metal contact may conform to at leastone ridge.

In general, in a fourth aspect, a method of forming a structure beginswith defining a recess with a maximum depth din a top surface of a (100)substrate. The substrate includes a first crystalline semiconductormaterial and the recess exposes a (111) surface of the substrate. AIII-nitride material having a lattice mismatch with the firstcrystalline semiconductor material is formed in the recess. The latticemismatch creates defects in the III-nitride material perpendicular tothe (111) surface, resulting in a substantially defect-free region inthe III-nitride material.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A depicts a schematic cross-sectional side view of a siliconsubstrate with a germanium layer formed thereon according to an“epitaxial necking” technique known in the art;

FIG. 1B is an XTEM image illustrating the semiconductor heterostructureof FIG.

FIGS. 2A-2C are schematic diagrams showing the three types ofcrystalline orientation for silicon;

FIGS. 3A-3B, 4A-4E, and 5A-5B depict schematic views of differentlattice-mismatched semiconductor heterostructures and structures forblocking dislocations therein, according to various embodiments of theinvention;

FIGS. 6A-6H and 7A-7C depict schematic cross-sectional side views of thelattice-mismatched semiconductor heterostructures having increasedactive area, according to various embodiments of the invention;

FIGS. 8, 9, 10A1-10A2, and 10B1-10B2 depict various applications ofdislocation blocking techniques for semiconductor devices, according tovarious embodiments of the invention;

FIGS. 11A-11B and 12A-12B depict Ge or III-V photodetector integrationinto Si substrate according to particular embodiments of the invention;

FIGS. 13A-13C depict semiconductor heterostructures employingdislocation-blocking techniques according to alternative embodiments ofthe invention; and

FIGS. 14A-14C, 15A-15D, and 16A-16D depict a method of trapping defectsby forming grooves in a semiconductor substrate in accordance withvarious embodiments of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In accordance with its various embodiments, the present inventioncontemplates fabrication of monolithic lattice-mismatched semiconductorheterostructures with limited area regions having upper surfacessubstantially exhausted of threading dislocations and other dislocationdefects, as well as fabrication of semiconductor devices based on suchlattice-mismatched hetero structures.

Silicon (Si) is recognized as presently being the most ubiquitoussemiconductor for the electronics industry. Most of silicon that is usedto form silicon wafers is formed from single crystal silicon. Thesilicon wafers serve as the substrate on which CMOS devices are formed.The silicon wafers are also referred to as a semiconductor substrate ora semiconductor wafer. While described in connection with siliconsubstrates, however, the use of substrates that include, or consistessentially of, other semiconductor materials, is contemplated withoutdeparting from the spirit and scope of the present invention.

In crystalline silicon, the atoms which make up the solid are arrangedin a periodic fashion. If the periodic arrangement exists throughout theentire solid, the substance is defined as being formed of a singlecrystal. If the solid is composed of a myriad of single crystal regionsthe solid is referred to as polycrystalline material. As readilyunderstood by skilled artisans, periodic arrangement of atoms in acrystal is called the lattice. The crystal lattice also contains avolume which is representative of the entire lattice and is referred toas a unit cell that is regularly repeated throughout the crystal. Forexample, silicon has a diamond cubic lattice structure, which can berepresented as two interpenetrating face-centered cubic lattices. Thus,the simplicity of analyzing and visualizing cubic lattices can beextended to characterization of silicon crystals. In the descriptionherein, references to various planes in silicon crystals will be made,especially to the (100), (110), and (111) planes. These planes definethe orientation of the plane of silicon atoms relative to the principlecrystalline axes. The numbers {xyz} are referred to as Miller indicesand are determined from the reciprocals of the points at which thecrystal plane of silicon intersects the principle crystalline axes.Thus, FIGS. 2A-2C show three orientations of the crystal plane ofsilicon. In FIG. 2A, the crystal plane of silicon intersects the x-axisat 1 and never intersects they or z-axis. Therefore, the orientation ofthis type of crystalline silicon is (100). Similarly, FIG. 2B shows(110) crystalline silicon and FIG. 2C shows (111) silicon. The (111) and(100) orientations are the two primary wafer orientations in commercialuse. Notably, for any given plane in a cubic crystal there are fiveother equivalent planes. Thus, the six sides of the cube comprising thebasic unit cell of the crystal are all considered (100) planes. Thenotation {xyz} refers to all six of the equivalent (xyz) planes.Throughout the description, reference will also be made to the crystaldirections, especially the <100>, <110> and <111> directions. These aredefined as the normal direction to the respective plane. Thus, the <100>direction is the direction normal to the (100) plane. The notation <xyz>refers to all six equivalent directions.

As discussed above, there is a need in the art for versatile andefficient methods of fabricating semiconductor heterostructures thatwould constrain substrate interface defects in a variety oflattice-mismatched materials systems. One conventional techniquementioned above that addresses control of threading dislocationdensities in highly-mismatched deposited layers, termed “epitaxialnecking,” is applicable only to devices with relatively small lateraldimensions. Specifically, in the prior art, metal oxide semiconductor(“MOS”) transistors are typically fabricated on (100) silicon waferswith the gates oriented such that current flows parallel to the <110>directions. Thus, for a FET device built on a (100) Si wafer with devicechannel orientation aligning with the <110> direction, both the channelwidth and channel length should be small compared to the height of aepitaxial necking mask, in order for the dislocations in alattice-mismatched semiconductor layer to terminate at a sidewall of themask on both directions. However, in modern CMOS circuits, the MOSFETdevice width often substantially exceeds the channel length, which, as aresult of CMOS scaling, is frequently very small. Accordingly, under theconventional necking approach, a number of dislocations will not beterminated at the sidewall of the mask in the direction of the channelwidth.

In contrast with the prior art approach of minimizing dislocationdefects, in its various embodiments, the present invention addresses thelimitations of known techniques, by utilizing greater thicknesses andlimited lateral areas of component semiconductor layers to producelimited-area regions having upper portions substantially exhausted ofdislocation defects. Referring to FIGS. 3A-3B, a substrate 310 isprovided that includes, or consists essentially of, a firstsemiconductor material, such as, for example, a group IV element, e.g.,germanium or silicon. The first semiconductor material may becrystalline. The substrate 310 may be, for example, a bulk siliconwafer, a bulk germanium wafer, a semiconductor-on-insulator (SOI)substrate, or a strained semiconductor-on-insulator (SSOI) substrate. Inone embodiment, the substrate 310 includes or consists essentially of(100) silicon. The substrate 310 may include a material having a firstconductivity type, e.g., n- or p-type, such as n+Si.

A dislocation-blocking mask 320 is disposed over the substrate. The maskhas an opening 325 extending to the surface of the substrate and definedby at least one sidewall 330. In various embodiments, the opening 325 isgenerally rectangular. The dislocation-blocking mask may include adielectric material, such as, for example, silicon dioxide or siliconnitride. At least a portion of the sidewall meets the surface of thesubstrate at an orientation angle a to a selected crystallographicdirection of the first semiconductor material. In addition, at least aportion of the sidewall is generally vertical, i.e. disposed at about 80to 120 degrees to the surface of the substrate, and, in a particularembodiment, substantially perpendicular to the surface of the substrate.

A regrowth layer 340 that includes a second semiconductor material isdeposited in the opening. In one embodiment, the selectedcrystallographic direction of the first semiconductor material isaligned with direction of propagation of threading dislocations in theregrowth layer. In certain embodiments, the orientation angle rangesfrom about 30 to about 60 degrees, for example, is about 45 degrees tosuch crystallographic direction. The surface of the substrate may have(100), (110), or (111) crystallographic orientation. In someembodiments, the selected crystallographic direction is substantiallyaligned with a <110> crystallographic direction of the firstsemiconductor material.

In various embodiments, the first semiconductor material may include, orconsist essentially of, silicon or a silicon germanium alloy. The secondsemiconductor material may include, or consist essentially of, a groupII, a group III, a group IV, a group V, and/or a group VI element,and/or combinations thereof, for example, selected from the groupconsisting of germanium, silicon germanium, gallium arsenide, aluminumantimonide, indium aluminum antimonide, indium antimonide, indiumarsenide, indium phosphide, and gallium nitride.

The regrowth layer can be formed in the opening by selective epitaxialgrowth in any suitable epitaxial deposition system, including, but notlimited to, atmospheric-pressure CVD (APCVD), low- (or reduced-)pressure CVD (LPCVD), ultra-high-vacuum CVD (UHVCVD), by molecular beamepitaxy (MBE), or by atomic layer deposition (ALD). In the CVD process,selective epitaxial growth typically includes introducing a source gasinto the chamber. The source gas may include at least one precursor gasand a carrier gas, such as, for example hydrogen. The reactor chamber isheated, such as, for example, by RF-heating. The growth temperature inthe chamber ranges from about 300° C. to about 900° C. depending on thecomposition of the regrowth layer. The growth system also may utilizelow-energy plasma to enhance the layer growth kinetics.

The epitaxial growth system may be a single-wafer or multiple-waferbatch reactor. Suitable CVD systems commonly used for volume epitaxy inmanufacturing applications include, for example, EPI CENTURAsingle-wafer multi-chamber systems available from Applied Materials ofSanta Clara, Calif., or EPSILON single-wafer epitaxial reactorsavailable from ASM International based in Bilthoven, The Netherlands.

In some embodiments, the regrowth layer is compositionally graded, forexample, includes Si and Ge with a grading rate in the range of >5%Ge/μm to 100% Ge/μm, preferably between 5% Ge/μm and 50% Ge/μm, to afinal Ge content of between about 10% to about 100% While the overallgrading rate of the graded layer is generally defined as the ratio oftotal change in Ge content to the total thickness of the layer, a “localgrading rate” within a portion of the graded layer may be different fromthe overall grading rate. For example, a graded layer including a 1 μmregion graded from 0% Ge to 10% Ge (a local grading rate of 10% Ge/μm)and a 1 μm region graded from 10% Ge to 30% Ge (a local grading rate of20% Ge/μm) will have an overall grading rate of 15% Ge/μm. Thus, theregrowth layer may not necessarily have a linear profile, but maycomprise smaller regions having different local grading rates. Invarious embodiments, the graded regrowth layer is grown, for example, at600-1200° C. Higher growth temperatures, for example, exceeding 900° C.may be preferred to enable faster growth rates while minimizing thenucleation of threading dislocations. See, generally, U.S. Pat. No.5,221,413, incorporated herein by reference in its entirety.

In a particular embodiment, the first semiconductor material is siliconand the second semiconductor material is germanium. In this embodiment,threading dislocations 350 in the regrowth layer propagate along a <110>direction, and lie at an angle of 45-degrees to the surface of the firstsemiconductor material. The dislocation mask having a generallyrectangular opening is disposed over the substrate such that thesidewall of the opening is disposed at a 45-degree angle to a <100>direction and is substantially aligned with a <110> crystallographicdirection. As a result of such orientation of the opening, dislocationswill reach and terminate at the sidewalls of the opening in thedislocation-blocking mask at or below a predetermined distance H fromthe surface of the substrate, such that threading dislocations in theregrowth layer decrease in density with increasing distance from thesurface of the substrate. Accordingly, the upper portion of the regrowthlayer is substantially exhausted of threading dislocations, enablingformation of semiconductor devices having increased channel width.

In certain versions of this and other embodiments of the invention, thesidewall of the opening in the dislocation-blocking mask has a height atleast equal to a predetermined distance H from the surface of thesubstrate. In these embodiments, the opening is substantiallyrectangular and has a predetermined width W that is smaller than alength L of the opening. For example, the width W of the opening can beless than about 500 nm, and the length L of the opening can exceed eachof W and H. In some versions of these embodiments, the substrateconsists essentially of silicon and has a (100) crystallographicorientation, the orientation angle is about 45 degrees to propagation ofdislocations in the regrowth layer, and the predetermined distance H isat least W√2. In other versions, the substrate consists essentially ofsilicon and has a (110) crystallographic orientation, the orientationangle is about 45 degrees, and the predetermined distance H is at leastW√6/3. In still other versions, the substrate consists essentially ofsilicon and has a (111) crystallographic orientation, the orientationangle is about 45 degrees, and the predetermined distance H is at least2 W.

In various embodiments of the invention, blocking of the dislocations ispromoted both by geometry and orientation of the mask discussed above aswell as because of the ‘image force’ whereby dislocations are attractedto substantially vertical surfaces, as explained in more detail below.In many embodiments, the image force alone is sufficient to cause theupper portion of the regrowth layer to be substantially exhausted ofthreading dislocations and other dislocation defects.

As skilled artisans will readily recognize, a dislocation near a surfaceexperiences forces generally not encountered in the bulk of a crystal,and, in particular, is attracted towards a free surface because thematerial is effectively more compliant there and the dislocation energyis lower. See Hull & Bacon, Introduction to Dislocations, 4th edition,Steel Times (2001). Image force is determined by material properties ofthe semiconductor being grown, as well as the distance between a givendislocation and the free surface. Thus, even when the dislocations havean orientation that does not favor trapping at sidewalls, the approachdiscussed above is still effective at certain dimensions because of theboundary forces that draw dislocations to free surfaces in order toreduce the elastic energy of the crystal. Mathematically, these forcesarise because the boundary conditions of the expressions for strainrequire strain components normal to a surface to be zero at thatsurface. Thus, force per unit of dislocation length on an edgedislocation, toward a vertical sidewall can be represented by theformula:

$F_{i} = \frac{{Gb}^{2}}{4\;\pi\;{d\left( {1 - v} \right)}}$WhereF_(i)=Image forceG=Shear modulusd=distance from free surfaceb=Burgers vectorν=Poisson's ratio

Referring to FIGS. 4A-4B, as used herein, the term “60° dislocation”refers to a dislocation for which the angle between the Burgers vectorand the dislocation line is 60°. These dislocations typically form indiamond-cubic or zincblende lattice-mismatched systems where the strainis relatively low (e.g. <2%). In the absence of forces on threads (whichcan come from other dislocations nearby or from a free surface nearby)they rise from the substrate surface at a 45° angle in <110> directions.However, when viewed from above (normal to the surface) they appear tolie in <100> directions.

Experimentally, it has been shown that for the case of germanium onsilicon (4% mismatch) dislocations within approximately 300 nm of a SiO₂sidewall are trapped. This is understood to be due to the influence ofthe image force. The angle between these dislocations and the sidewallappears to range between approximately 45-55.

The relevant material constants for Ge are:

G=4.1e11 dyne/cm²

v=0.26; and

b=3.99 Å

Based on the above formula and the experimental observation that ford≤300 nm dislocations in Ge on Si are bent toward an SiO₂ sidewall, theforce necessary to bend a dislocation in a cubic semiconductor crystaltoward a free surface is approximately 2.3 dyne/cm. Thus, distance fromfree surfaced for other materials can be estimated with certain degreeof accuracy based on their known values for G, v, and b. For example, bythese calculations:

For GaAs d = 258 nm For InP d = 205 nm For AlSb d = 210 nm For InSb d =164 nm

Referring to FIG. 4C, for full trapping, the hole or trench lateraldimension w is preferably less than or equal to approximately 2*d, whilethe vertical dimension h is preferably at least approximately d, where dis calculated discussed above. These criteria are expected to besubstantially independent of the orientation of the boundary of thesidewall and the substrate surface. Thus, in various embodiments of theinvention, dislocations in the lower portion of the regrowth layer aretrapped by employing a dislocation-blocking mask with an elongatedopening, e.g. a trench, having a width calculated as discussed above andoriented without any regard for the direction of propagation ofdislocations in the regrowth layer.

Further, as shown in FIG. 4D and used herein, the term “90° dislocation”refers to a dislocation for which the angle between the Burgers vectorand the dislocation line is 90°. These dislocations primarily form inmismatched systems where the strain is relatively high (e.g. >2%). Inthe absence of forces on threading dislocation (which can come fromother dislocations nearby or from a free surface nearby) they rise fromthe substrate surface at a 90° angle in <100> directions. Thus, thesedislocations can be trapped most optimally by using adislocation-blocking mask with slanted, rather than vertical sidewalls,as shown in FIG. 4E.

The following summarizes mechanisms for trapping dislocations indifferent kind of diamond-cubic or zincblende semiconductorheterostructures:

1. Low mismatch, low image force

-   -   60° dislocations predominate    -   Threads lie in <110> directions, rising from surface at 45°    -   Best approach for trapping dislocations is to rely on        appropriate orientation of sidewalls and appropriate        dimensioning of openings, as described above in connection with        FIGS. 3A-3B;

2. Low mismatch, high image force

-   -   60° dislocations predominate    -   Threads bend toward free substantially vertical surfaces    -   Best approach for trapping dislocations is described above in        connection with FIGS. 4A-4C;

3. High mismatch, high image force

-   -   90° dislocations predominate    -   Threads bend toward free substantially vertical surfaces    -   Best approach for trapping dislocations is described above in        connection with FIGS. 4A-4C;

4. High mismatch, low image force

-   -   90° dislocations predominate    -   Threads lie in <110> directions, rising from surface at 90°    -   Best approach for trapping dislocations is described above in        connection with FIGS. 4D-4E;

Hexagonal semiconductors, such as the III-nitride (III-N) materials, areof great interest for high-power high-speed electronics andlight-emitting applications. For epitaxy of hexagonal semiconductorssuch as III-nitrides on Si, the (111) surface of Si is commonlypreferred over the (100). This is because the (111) surface of Si ishexagonal (even though Si is a cubic crystal). This makes a bettertemplate for hexagonal crystal growth than the cubic (100) face.However, as mentioned above, epitaxial necking approach discussed aboveis less effective in these applications, because the threadingdislocations in the hexagonal semiconductors disposed over thelattice-mismatched Si (111) substrates may not be effectively confinedby the vertical sidewalls because the threading dislocations in suchmaterials typically have a different orientation relative to thesubstrate, compared to the more commonly used cubic semiconductors, suchas Si, Ge, and GaAs For example, as described above in connection withFIG. 4E, for certain surface orientations of substrate and crystallinestructure of lattice-mismatched regrowth region, the threading defectstend to propagate perpendicular to the substrate, which may not favortrapping by vertical sidewalls of the dislocation-blocking mask. This isthe case when GaN is grown on the (100) surface of Si. In such a case,in some embodiments, the angle of the sidewalls of the opening can beslanted relative to the substrate, as shown in FIG. 4E such thatvertically propagating defects intersect the angled sidewalls.

In other embodiments, the surface of the underlying substrate itselfexposed in the opening is configured to enable confinement of thethreading dislocations. Referring to FIG. 5A, after thedislocation-blocking mask is disposed over the Si (100) substrate and anopening is defined there through, an etch that is selective to the (111)crystallographic plane of Si, for example, a KOH solution, is applied tothe portion of the substrate exposed at the bottom of the seed window toexpose (111) surfaces. A lattice-mismatched semiconductor material isthen deposited in the opening over the substrate, and the epitaxialdeposition continues such that a heteroepitaxial region is grown overthe material disposed in the opening, laterally expanding over the mask.Because of the configuration of the underlying surface, orientation ofthe threading dislocations in the heteroepitaxial region is atapproximately 45° to the surface of the substrate, facilitating trappingof the dislocation by substantially vertical sidewalls of the mask, asshown in FIG. 5B. Then, if small areas of hexagonal semiconductormaterial are desired for device active areas, the heteroepitaxialovergrowth regions expanding from the individual openings can beplanarized (e.g. via CMP), to be substantially co-planar with theadjacent insulator areas. Alternatively, if a large area is desired,growth can proceed until neighboring regions coalesce, followedoptionally by planarization of the resulting structure. In the lattercase, because lateral growth rates of hexagonal semiconductor can bedramatically increased over growth rate normal to the surface employingvarious known approaches, these semiconductor materials afford processflexibility not available with cubic semiconductors grown on (100)surfaces. Specifically, differential growth rates of these materialsallows for widely-spaced seed trenches; for example, spacing may be fivetimes trench width or even greater, offering a substantial advantageover closely-spaced seed trenches, if the defects which are known toform when epitaxial growth fronts coalesce cannot be substantiallyeliminated.

FIGS. 6A-6F depicts schematic cross-sectional side views of thelattice-mismatched semiconductor heterostructures having increasedsurface area according to various embodiments of the invention.Specifically, as discussed in more detail below, the area of the upperportion of the lattice-mismatched heterostructure substantiallyexhausted of threading dislocations is increased, compared to theembodiments described above with reference to FIGS. 3A-3B. For example,as described in more detail below, in some embodiments, the opening inthe dislocation-blocking mask has a variable width. In other versions,the sidewall of the opening in the dislocation-blocking mask includes afirst portion disposed proximal to the surface of the substrate, and asecond portion disposed above the first portion. A height of the firstportion can be at least equal to a predetermined distance H from thesurface of the substrate, where the threading dislocations terminate atthe sidewall of the opening in the dislocation-blocking mask at or belowthe distance H. In some versions of these embodiments, the first portionof the sidewall can be substantially parallel to the second portion.Also, in some versions, the second portion of the sidewall is flaredoutwardly.

In many of the embodiments described below, a substrate 510 includes, orconsists essentially of, silicon. The regrowth layer includes, orconsists essentially of, a semiconductor material that is one of a groupII, a group III, a group IV, a group V, and/or a group VI elements,and/or combinations thereof, for example, selected from the groupconsisting of germanium, silicon germanium, gallium arsenide, aluminumantimonide, indium aluminum antimonide, indium antimonide, indiumarsenide, indium phosphide and gallium nitride. A dislocation-blockingmask 520 having an opening therein is disposed over the substrate. Thedislocation-blocking mask may include a dielectric material, such as,for example, silicon dioxide or silicon nitride. At least a portion ofthe sidewall meets the surface of the substrate at an orientation angleα to a selected crystallographic direction of the first semiconductormaterial. A regrowth layer 540 that includes a second semiconductormaterial is deposited in the opening. In various embodiments, theselected crystallographic direction of the first semiconductor materialis aligned with direction of propagation of threading dislocations inthe regrowth layer. In various embodiments, the orientation angle rangesfrom about 30 to about 60 degrees, for example, is about 45 degrees. Asmentioned above, in many embodiments of the invention, blocking of thedislocations is promoted by geometry and orientation of the maskdiscussed above and/or the ‘image force.’

Referring to FIG. 6A, in one embodiment, the dislocation-blocking maskis formed by depositing a first low-temperature oxide layer 521 havingthickness h₁ over the substrate. The thickness h₁ is selected to be atleast equal to the distance from the surface of the substrate at whichthe threading dislocations (and/or other dislocation defects such asstacking faults, twin boundaries, or anti-phase boundaries) terminate atthe sidewall of the opening in the dislocation-blocking mask, asdiscussed above. A first aperture having a diameter d1 or a first trenchhaving a width w1, both the width w1 and diameter d1 being smaller thanthe thickness h₁, are formed in the layer 521 by a conventionalmasking/etching technique. After the mask is stripped, a secondlow-temperature oxide layer 522 having a thickness h2 is deposited overthe layer 521. Then, a second aperture of diameter d2 or a second trenchhaving a width w2 is formed in the layer 522 by a conventionalmasking/etching technique, such that w1<w2 (or d1<d2). After the mask isstripped, the regrowth layer of second semiconductor material isdeposited in the first and second apertures or in first and secondtrenches by selective epitaxy, according to any of the techniquesdisclosed in U.S. Patent Application Publication No. 2004/0045499A byLangdo et at., incorporated herein by reference. As discussed above,following deposition, threading dislocations and/or other dislocationdefects substantially terminate in the first aperture (or in the firsttrench) at or below thickness h₁. As a result, the regrowth layerportion having thickness h2 that is substantially exhausted of threadingdislocations is obtained with an upper surface that is larger comparedto the embodiments described above with reference to FIGS. 3A-3B.

Referring to FIG. 6B, in another embodiment, an overgrowth layer 555that includes the second semiconductor material is deposited over theregrowth layer 540 and over a portion of the dislocation-blocking mask520 adjacent to the regrowth layer. At least a portion of the overgrowthlayer may be deposited as non-crystalline (i.e. amorphous) material andcan be crystallized subsequently, for example by an anneal step at atemperature higher than the deposition temperature. Thus, in thisembodiment, crystallization of the overlayer is used to create crystalmaterial in the overlayer regions over the regrowth layer, which isamorphous upon deposition. The arrows in FIG. 6B indicate a crystallizedregion expanding outward from the opening in the dislocation blockingmask, as amorphous material which may form at least a portion of theovergrowth layer 555 is crystallized.

Referring to FIG. 6C, in yet another embodiment, deposition of theregrowth layer in the opening of the dislocation-blocking mask isfollowed by a step of lateral epitaxial deposition to increase theuseful surface area. It may also utilize the higher growth rates typicalof (100) surfaces compared to (110) or (111) surface to increase lateralovergrowth in this embodiment. For example, the overgrowth regions canbe used as source/drain areas which typically have less stringentmaterial quality requirement than the channel material.

Referring to FIG. 6D, in still another embodiment, the useful upper areaof the regrowth layer 540 is increased by gradually increasing the sizeof the regrowth region. Similar to the embodiment described above withreference to FIG. 6A, the dislocation-blocking mask includes twolayers—a first layer having thickness h₁, and a second layer havingthickness h2. The thickness h₁ is selected to be at least equal to thedistance from the surface of the substrate at which the threadingdislocations and/or other dislocation defects terminate at the sidewallof the opening in the dislocation-blocking mask, as discussed above.That is, a first aperture having a diameter d1 or a first trench havinga width w1, both the width w1 and diameter d1 being smaller than thethickness h₁, are formed in the layer 521 by a conventionalmasking/etching technique. After the mask is stripped, a second lowtemperature oxide layer 522 having a thickness h2 is deposited over thelayer 521. Then, a second aperture of diameter d2 or a second trenchhaving a width w2 is formed in the layer 522 by a conventionalmasking/etching technique, such that w1<w2 (or d1<d2). In contrast tothe embodiment depicted in FIG. 6A, however, the width w2 of the secondtrench is gradually increased such that the sidewall of the trench, i.e.the opening in the layer 52 x 2, gradually flares outwardly. This effectcan be achieved, for example, by conventional masking/etching techniqueswherein the etchant and masking material are chosen such that themasking material is eroded laterally during the etching process,gradually exposing more of the dislocation-blocking mask below,resulting in an opening in the dislocation-blocking mask that flaresoutward. For example, the masking material could be conventionalphotoresist and the etchant could be a mixture of the gases CF₄ and H₂,used in a conventional RIE system. After the mask is stripped, theregrowth layer of second semiconductor material is deposited byselective epitaxy in the opening defined by the layers 521, 522. Asdiscussed above, following deposition, threading dislocations (and/orother dislocation defects such as stacking faults, twin boundaries, oranti-phase boundaries) substantially terminate in the first aperture (orin the first trench) at or below thickness h₁. Thus, in this embodiment,the dislocations are terminated in the first portion of the regrowthregion at or below thickness h₁, and then the regrowth layer becomeslarger and larger gradually, allowing for high-quality epitaxial growthwith large surface area for large device fabrication.

Referring to FIG. 6E, in an alternative version of the embodimentdiscussed above in connection with FIG. 6D, a dislocation-blocking maskhaving an opening with outward slanted sidewalls, i.e. the structurethat is substantially narrower at the bottom than the top, can be formedwith only one lithography step, followed by spacer deposition and etch.This technique is generally more economical and may overcomelithographic alignment problems, or lithographic minimum featurelimitations, inherent with the lithography-and-etch approach. Thespacers can be formed from the same or different material than theinsulator layer. For either case, selective epitaxial growth followscreation of the opening or trench.

FIGS. 6F-6H show further techniques to increase the surface area.Referring to FIG. 6F (as well as, again, to FIG. 6B), in one embodiment,silicon nitride is utilized instead of silicon dioxide as a dielectricmaterial for the dislocation-blocking mask 520 that defines two openings535. After the regrowth regions 540 are epitaxially grown in theopenings, overgrowth regions 560 are deposited thereover. Using siliconnitride facilitates merging two overgrown regions on the surface ofdislocation-blocking mask 520 layer with fewer defects, resulting inlarger surface area. Referring to FIG. 6G, in one particular version ofthe embodiment of FIG. 6F, a layer of second semiconductor material 570is deposited over the substrate 510 before forming thedislocation-blocking mask 520 thereon, such that the regrowth regions540 merge at the top of the dislocation-blocking mask with pre-definedlattice spacing. This lattice spacing in the regrowth regions followsthe lattice spacing of the layer 570 and thus it has less latticemisalignment when two epitaxy structures merge. Referring to FIG. 6H, inanother version of the embodiment of FIG. 6F, the dislocation-blockingmask defines two or more closely spaced flared openings, such that ahorizontal top surface of the mask is minimized or, in certainimplementations, eliminated. In this version, the lateral overgrowthregion, often prone to defects, is negligible or altogether absent,thereby improving the quality of the resulting merged overgrowth region.

Further, referring to FIGS. 7A-7C, in some embodiments, the inventionfocuses on creating large active areas within the heteroepitaxial regionby a combination of epitaxial necking and ELO techniques, employing aself-assembled dislocation-blocking mask. Specifically, an dielectriclayer defining an array of openings therethrough can be formed usingself-assembly techniques, thereby avoiding traditional time-consuminglithography and etch approaches. For an example of how a self-assembledarray of vertical openings in an insulator layer could be created on aSi substrate, see an article by Wenchong Hu et at. entitled “Growth ofwell-aligned carbon nanotube arrays on silicon substrates using porousalumina film as a nanotemplate,” published in Applied Physics Letters,Vol. 79, No. 19 (2001) and incorporated herein by reference, describinghow anodic oxidation of the aluminum can be used to create aself-assembled array of vertical openings similar to that shown in FIG.7A-7B, where the insulator is alumina (Al₂O₃). The process described byHu et at., however, leaves a small residual layer of alumina at thebottom of each hole. To remove this residual layer, an anisotropic dryetch (much higher etch rate normal to the wafer surface than parallel tothe wafer surface) could be performed, exposing the silicon which is the‘seed’ for subsequent epitaxial necking. Then, heteroepitaxial regionsare selectively grown within and out of the openings, at least untilresulting overgrowth regions coalesce. Depending on lateral dimensionsof the aperture, degree of mismatch, and rigidity of sidewall oxide,either plastic or elastic relaxation of the heteroepitaxial “pillars”may dominate. The resulting heteroepitaxiallayer is then planarized(FIG. 7C), e.g. via CMP, and the active-area, substantially exhausted ofthreading dislocations and/or other dislocation defects is used fordevice fabrication.

FIGS. 8-10 depict various applications of dislocation-blockingtechniques according to various embodiments of the invention forfabrication of CMOS devices. FIG. 8 shows various device structuresdisposed over regrowth or overgrown regions fabricated according to theinvention, such as MOSFET devices including Ge, lnGaAs, strained Ge/SiGeand other materials, or HEMT devices, e.g. including lnGaAs The startingsubstrate can be Si substrate or SOI/SSOI substrate. In one example,n-FET and p-FET digital devices are fabricated on a SSOI substrate,while RF/analog devices are fabricated over a Ge region grown over theSi substrate using the approaches discussed above. By integratingadvanced materials into Si substrate, electron and hole mobility can beenhanced significantly. In order to avoid the deleterious effects ofdislocations defects on such FET devices, the channel, source, and drainregion should be confined to an upper region of regrowth or overgrownmaterial which is substantially defect-free. As discussed in detailabove, blocking of the threading dislocations and other defects ispromoted by geometry and orientation of the mask and/or the image force.In many embodiments, the image force alone is sufficient to cause theupper region of the regrowth or overgrown material to be substantiallyexhausted of threading dislocations and other dislocation defects.

Furthermore, still referring to FIG. 8, a wide bandgap material whichwill suppress junction leakage (such as AlSb) can be used for initialgrowth, followed by a material with high electron mobility for the FETchannel (such as lnAs). In this embodiment, preferably, the twosemiconductor materials have similar lattice constants, to reduce thepossibility of dislocations forming at the interface between them. Alsoin this embodiment, the growth of the wide bandgap material may befollowed by a planarization step so that its surface is substantiallyplanar with the top of the dislocation blocking mask; subsequently athin layer of the high-mobility material can be grown to accommodate theMOS channel. Preferably, the bottom of the FET junctions is disposedwithin the wide bandgap region to suppress junction leakage.

FIG. 9 depicts another application of the dislocation-blockingtechniques according to various embodiments of the invention in CMOS.This method allows the Ge/III-V necking technique to be used inrelatively large CMOS devices. When applying the dislocation-blockingtechnique in a CMOS device as in the embodiment of FIG. 8, the length ofdevice active region L_(active) should be small enough to satisfy theaspect ratio requirement discussed above. L_(active), which includessource/drain lengths as well, is, however, much larger than the devicechannel length Lg. The embodiment shown in FIG. 9 addresses a situationwhere Ge or GaAs growth is performed at a narrow channel region only;source/drain materials are then deposited separately. This approachallows for Ge or GaAs growth techniques to be applied to much largerdevices, for example, 90 nm node CMOS devices instead of 22 nm nodedevices. This channel-only Ge/III-V dislocation-blocking approach mayalso be combined with other desirable source/drain engineeringtechniques, such as raised source/drain techniques, Schottkysource/drain approaches, or the use of materials on the source/drainregion different from the material in the channel region forsource/drain dopant/conductivity optimization. The quasi source/drain“on-insulator” structure also reduces the junction capacitance. Properdeposition of source/drain materials may also introduce localized strainin the channel region for mobility enhancement purpose. The approachdiscussed above can be applied to pre-defined small channel regionsonly. The epitaxial deposition in the source/drain regions may bedefective, but as long as the dislocations terminate on the sidewalls ofthe narrow channel region, the defect density in source/drain isacceptable.

Besides the conventional planar MOSFETs, the dislocation-blockingtechnique of the invention can also be used to fabricate non-planarFETs. As mentioned above, blocking of the threading dislocations andother defects is promoted by geometry and orientation of the mask and/orthe image force. In many embodiments, the image force alone issufficient to cause the upper region of the regrowth or overgrownmaterial to be substantially exhausted of threading dislocations andother dislocation defects. FIGS. 10A1, 10A2, 10B1, and 10B2 showbody-tied finFETs or tri-gate transistor structures which takes theadvantage of the vertical shape of the lattice-mismatched material.FIGS. 10A1-10A2 illustrate a first exemplary method of forming finFETsor tri-gate transistor structures. FIG. 10A1 illustrates depositing orgrowing an oxide layer, followed by depositing a nitride layer, maskingand etching a trench of width w<0.5 h; (the trench orientation may be ina <110> direction, so all the threading dislocations along <110>directions (which will lie at an angle of 45-degrees to the surface ofthe first semiconductor material) will intersect sidewalls within theheight of h); selectively growing Ge or III-V in the trench; andchemical-mechanical polishing to remove the portion of selective growthoutside of the trench FIG. 10A2 illustrates selectively removingnitride, which results in fin structures; and then growing and/ordepositing insulator material around the fin structures; followed bydepositing, masking and etching gate electrodes and ion implantation tocreate source/drain regions. FIGS. 10B1-10B2 illustrates a secondexemplary method of forming finFETs or tri-gate transistor structures.FIG. 10B1 illustrates depositing or growing an oxide layer, masking andetching a trench of width w<0.5 h; selectively growing Ge or III-V inthe trench; and chemical-mechanical polishing to remove the portion ofselective growth outside of the trench. FIG. 10B2 illustratesselectively removing a portion of the oxide, which results in finstructures; and then growing and/or depositing insulator material aroundthe fin structures; followed by depositing, masking and etching gateelectrodes and ion implantation to create source/drain regions.

Besides FET devices, the dislocation-blocking techniques of theinvention can also be used to fabricate other types of devices, such asoptical devices. Referring to FIGS. 11A-11B and 12A-12B, in someembodiments, Ge or III-V photodetectors are integrated into a Sisubstrate using such techniques. In an exemplary embodiment shown inFIGS. 11A and 11B (FIG. 11A is a side view and FIG. 11B is a top view),a lower contact is implanted on a Si substrate to form p+-type region;low-temperature oxide is deposited; apertures or trenches are etchedthrough the low-temperature oxide layer to explore the Si substrate; andGe or III-V materials are selectively grown on the apertures or trencheswithin-situ doping until past the defect regions (p-type). Further,epitaxial growth continues until the thickness is sufficient to allowfor desirable levels of absorption of incident light, and then the toplayer is implanted to form an n-type region. In another configuration,the light comes from the side (e.g. from in-plane waveguide) instead offrom the top, as shown in FIGS. 12A and 12B (FIG. 12A is a side view andFIG. 12B is a top view). This allows light detection to occur in-planewith the wafer surface and also to allow growth thickness to beindependent of absorption depth.

In various embodiments described above, the dislocation-blocking isperformed in a vertical direction. FIG. 13A shows an alternativeembodiment where the dislocation-blocking may conduct in a lateraldirection, for example from the source or drain region. Therefore, thedevice can be an SOI structure. In one embodiment, the gate oxide andgate stack can be formed first, before the dislocation-blocking growthunder the gate, using a channel-replacement-type process. This approachaddresses the self-alignment issue and any surface roughness issues.

FIG. 13B shows another method which allows dislocations be terminatedfor a large size epitaxial area. The method includes two steps ofepitaxial growth, which take different growth directions, so that thedislocations in one direction terminate at the sidewall during the firstepitaxial growth, and the dislocations in another direction, which mayhave large device dimensions, can terminate on the sidewall when theepitaxial growth changes the direction.

Conventional Ge/III-V necking forms crystal material in the verticaldirection. Therefore, when building planar MOS or finFET type devices onthat crystal, the device is typically a bulk-type or body-tied, not an“on-insulator” structure. Bulk-type of Ge or GaAs FET may exhibit largejunction leakage and poor short-channel effect control. One solution isto build the device vertically instead of parallel to horizontalsurface. FIG. 13C shows one embodiment of such structure: avertical-channel FET, which incorporates the benefits that a verticalFET has, for example, SCE control, better scalability, etc. Anotherapproach is to epitaxially grow an oxide layer that is lattice-matchedto the second semiconductor material during selective deposition of thesecond semiconductor material. As result, there is an oxide layer withinthe regrowth region underlying a portion thereof subsequently used fordevice fabrication, as discussed in more detail in co-pending U.S.patent application Ser. No. 11/000,566 by Currie, incorporated herein byreference.

FIGS. 14-16 illustrate a method, in accordance with embodiments of thecurrent invention, for reducing defects in lattice-mismatchedsemiconductor heterostructures by forming a recess or groove in asubstrate, and also illustrate devices formed in accordance with themethod. FIG. 14A shows a semiconductor structure 1400 that includes asubstrate 310 and a dielectric layer 1404 disposed thereover. Asdiscussed previously, with reference to FIGS. 3A-3B, the substrate 310may include, or consist essentially of, a first semiconductor material,such as, for example, a group IV element, e.g., germanium or silicon.The first semiconductor material may be crystalline. The substrate 310may be, for example, a bulk silicon wafer, a bulk germanium wafer, asemiconductor-on-insulator (SOI) substrate, or a strainedsemiconductor-on-insulator (SSOI) substrate. In one embodiment, thesubstrate 310 includes or consists essentially of (100) silicon. Thesubstrate 310 may include a material having a first conductivity type,e.g., n- or p-type, such as n⁺ Si.

The dielectric layer 1404 may be formed over the substrate 310. Thedielectric layer 1404 may include or consist essentially of a dielectricmaterial, such as silicon nitride or silicon dioxide (S_(i)O₂). Thedielectric layer 1404 may be formed by any suitable technique, e.g.,thermal oxidation or plasma-enhanced chemical vapor deposition (PECVD).In some embodiments, the height h₁ of the dielectric layer 1404 may bein the range of, e.g., 25-1000 nm. In a preferred embodiment, the heighth1 is approximately 600 nm.

An opening or trench 1406 may be formed in the dielectric layer 1404,exposing a portion 1408 of the surface of the substrate 310. More thanone opening 1406 may be formed, and each opening 1406 may have a heightequal to the height of the dielectric layer, e.g., height h1, and awidth w1. The opening(s) 1406 may be created by forming a mask, such asa photoresist mask, over the substrate 310 and the dielectric layer1404. The mask may be patterned to expose a portion of the dielectriclayer 1404. The exposed portion of the dielectric layer 1404 may beremoved by, for example, reactive ion etching (RIE) to define theopening 1406. The opening 1406 may be defined by at least one sidewall1407. In one embodiment, the opening 1406 is formed within the substrate310, and the dielectric sidewall 1407 is formed within the opening 1406.

The opening 1406 may be substantially rectangular in terms ofcross-sectional profile, a top view, or both. With respect to a topview, the width w₁ may be smaller than the length l₁ (not shown) of theopening. For example, the width w1 of the opening 1406 may be less thanabout 500 nm, e.g., about 10-500 nm, and the length l₁ of the opening1406 may exceed w₁. The ratio of the height h1 of the opening to thewidth w1 of the opening 1407 may be ≥0.5, e.g., ≥1. The opening sidewall1407 may not be strictly vertical.

Referring to FIG. 14B and also to FIG. 5A, a portion 1408 of the surfaceof the substrate 310 may be removed by, e.g., etching, to form a recessor groove 1410. The recess 1410 may be formed by wet etching thesubstrate 310 with, for example, KOH or NaOH, or by dry etching, such asplasma etching. In one embodiment, the surface 1412 in the recess 1410features non-(100) surfaces, such as (111) semiconductor surfaces, e.g.,(111) Si surfaces. The recess 1410 may have a maximum depth dcorresponding to its deepest point farthest from the substrate surfaceand may have a v-shaped profile. The ratio of the height of thedielectric layer 1404 plus the maximum depth of the recess 1410 to thewidth of the opening may be greater than or equal to one, i.e.,(h₁+d₁)/w₁≥1.

In one embodiment, and with reference also to FIG. 6H, another portionof the dielectric layer 1404 may be partially removed with, for example,a hydrofluoric acid etch. A portion of the dielectric layer 1404 distalto the substrate 310 may be removed at a faster rate than a portionproximate the substrate 310, creating a non-vertical sidewall 1414. Theremaining portion of the dielectric layer 1404 may resemble an invertedV when viewed in cross-section. In one embodiment, the sidewall 1414 issubstantially parallel to the surface 1412 of the opening 1410. Inanother embodiment, a horizontal portion 1416 of the dielectric layer1404 remains after the etch, and the width w₂ of the horizontal portion1416 is much less than the width co of the opening 1406.

FIG. 14C shows an alternative embodiment of the current invention inwhich the dielectric layer 1404 is removed from the substrate 310 afterthe formation of the recess (es) 1410. The dielectric layer 1404including SiO₂ may be removed with, for example, a hydrofluoric acidetch. This embodiment of the invention may be suitable for the growth ofIII-nitride semiconductor layers or layers of other semiconductormaterials having hexagonal lattice structures. Any dislocation defectsthat form in a III-nitride material (or other suitable material)deposited on the surface of the substrate 310 may form perpendicularlyto the opening surfaces 1412, rather than vertically, as the defects maytypically form. The non-vertical formation of the defects may,therefore, create defect-free regions in the deposited material. In analternative embodiment, the recesses 1410 improve the formation of GaAs(or other semiconductor materials having cubic or zinc blende latticestructures) heteroepitaxial structures.

FIG. 15A illustrates an embodiment of the current invention in which thedielectric layer 1404 is not removed from the substrate 310 after theformation of the recess (es) 1410. A second crystalline semiconductormaterial 1500, which is lattice-mismatched to the first crystallinesemiconductor material in the substrate 310, may be formed within therecess 1410. In one embodiment, the second crystalline semiconductormaterial 1500 may be further formed within the opening 1406. The secondcrystalline semiconductor may be formed by, for example, metal-organicchemical vapor deposition (MOCVD) or molecular-beam epitaxy (MBE). Thesecond crystalline semiconductor material 1500 may be a III-V material,such as GaAs or lnP, a type-IV material, such as Ge or SiGe, or an alloyor mixture including any of these materials, such as lnGaP. In oneembodiment, an etch-stop layer (not shown), including a wide band-gapmaterial, may be formed on top of the second crystalline semiconductormaterial 1500. Dislocation defects 1502 may form in the secondcrystalline semiconductor material 1500 near the interface 1504 betweenthe substrate 310 and the second crystalline semiconductor material1500. The defects may form along the (111) direction, which may beparallel to the surface 1412 of the substrate 310. The defects mayterminate at a height H₁ above the deepest point of the recess 1410. Inone embodiment, the defects terminate within the recess 1410, and H₁≤d₁.In another embodiment (e.g., the embodiment illustrated in FIG. 15A),the defects terminate within the opening 1406, and H₁≤h₁+d₁. In a thirdembodiment, the defects terminate at a height h₁ that is less than orequal to the width w1.

The recess 1410 may effectively increase the height h of the opening1406. The surfaces 1412 along the interface 1504 may define an angle1501 with the horizontal of approximately 57 degrees. The depth d maythus be equal to tan(57°) w₁/2, and the effective height may be equal toh₁+tan(57°)×w₁/2. The height h₁ may be effectively increased regardlessof the material to be grown in the opening 1406. In one embodiment, therecess 1410 allows a reduction in the height h₁ because the effectiveincrease of h₁ may cause any dislocation defects to terminate at a lowerheight above the substrate 310.

In one embodiment, the second crystalline semiconductor material 1500does not extend above the height h of the dielectric layer 1404. In analternative embodiment, the second crystalline semiconductor material1500 extends above the height h of the dielectric layer 1404, and maycoalesce with the second crystalline semiconductor material grown in aneighboring opening 1406 to form a single layer of the secondcrystalline semiconductor material 1500 above the dielectric layer 1404.

In one embodiment, a buffer layer 1503, comprising a third crystallinesemiconductor material, is formed between the second crystallinesemiconductor material 1500 and the substrate 310. The buffer layer maybe formed on the surface 1412 of the substrate 310, and extendapproximately up to the dielectric layer 1404. In another embodiment,the buffer layer 1503 is confined to the recess 1410. The boundarybetween the second 1500 and third 1503 crystalline semiconductormaterials may be proximate the boundary defined by the interface betweenthe exposed portion of the substrate 310 and the dielectric sidewall1407. The buffer layer 1503 may be used to facilitate the formation ofthe second crystalline semiconductor material 1500 if there is a largedifference between the lattice constants of the second crystallinesemiconductor material 1500 and of the substrate 310. For example, thesubstrate 310 may include Si and the second crystalline semiconductormaterial 1500 may include lnP, so that the two materials differ inlattice constants by approximately eight percent. In this example, GaAsmay be used as the buffer layer, because its lattice constant differsfrom that of both Si and lnP by approximately four percent. In anotherembodiment, Ge or another material having a lattice mismatch to thefirst and/or second crystalline semiconductor materials of less thaneight percent may be used as a buffer layer.

In one embodiment, the buffer layer 1503 may include a constantconcentration of the third crystalline semiconductor material, or theconcentration may vary such that the lattice constant of the bufferlayer 1503 is closer to that of the substrate 310 at the bottom of thebuffer layer and closer to that of the second crystalline semiconductormaterial 1500 near the top of the buffer layer. In another embodiment,multiple buffer layers may be used. The use of one or more buffer layersmay allow the formation of one or more heteroepitaxial material layerswith large lattice-constant mismatches, while reducing the height h ofthe dielectric layer 1404 and/or depth d of the recess 1410. Theheteroepitaxial material layers may be formed inside the openings 1406or above the dielectric layer 1404.

FIG. 15B illustrates that, in one embodiment, the second crystallinesemiconductor material 1500 may be planarized by, for example, CMP. Asubstantially smooth surface 1506 may be formed as a result of theplanarization. The upper portion 1508 of the second crystallinesemiconductor material 1500 may be substantially free of defects.

FIG. 15C shows, in one embodiment of the current invention, a devicestructure 1510 formed on the surface 1506 of the second crystallinesemiconductor material 1500. In an alternative embodiment, the devicestructure 1510 is wholly or partially formed within the opening 1406.The device structure 1510 may include a diode; a transistor; a photonicdevice such as a photovoltaic device, LED, or laser diode; or any otherpassive or active semiconductor device. The device structure 1510 mayinclude a single layer of a semiconductor material, or may include morethan one layer. Each layer may include or consist essentially of atype-IV semiconductor material or a III-V semiconductor material. In oneembodiment, semiconductor devices may be formed in regions of thesubstrate 310 proximate the device structure 1510.

FIG. 15D depicts, in one illustrative embodiment, a flip-chip waferbonding process. A thin metal layer 1512 may first be formed on top ofthe device structure 1510. The thin metal layer 1512 may serve as bothan electrical conductor and, in the case where the device structure 1510is a photonic device, as an optical reflector. In various embodiments,the thin metal layer is approximately 100-200 nm thick and includes orconsists essentially of aluminum (for photonic devices emitting visiblelight) or gold or silver (for infrared light). The semiconductorstructure 1400 may then be bonded to a handle wafer 1514 to define abonded structure 1515. The handle wafer 1514 may include or consistessentially of a semiconductor material such as Si. Finally, a topcontact layer 1516 may be formed on a top surface of the handle wafer1514 and a bottom contact layer 1518 may be formed on a bottom surfaceof the substrate 310.

FIG. 16A illustrates a flipped structure 1600, which may be the bondedstructure 1520 rotated by 180 degrees. The substrate 310 may be removedwith, for example, an etching process, exposing surfaces 1602 of thesecond crystalline semiconductor material 1500. The defects 1502 may nowbe near the exposed surfaces 1602 of the second crystallinesemiconductor material 1500. The exposed portion of the secondcrystalline semiconductor material 1500 may include a non-planarsurface.

The wafer bonding and flipping process may present advantages during theformation of the second crystalline semiconductor material 1500, becauseany layer or layers that include the second crystalline semiconductormaterial 1500 may ultimately be rotated 180 degrees. For example, withreference to FIG. 15A, a layer with a high bandgap may first bedeposited on the substrate 310 first, then a layer with a mediumbandgap, and finally a layer with a low bandgap. Because the first layerwith a high bandgap may require a higher growth temperature than thematerials with lower bandgaps, depositing the layers in this order mayspare the low bandgap layer from being subjected to the high temperaturerequired for the high bandgap layer. The low bandgap layer may require alower growth temperature, which the high bandgap material may be betterable to withstand. When the device structure 1400, including theillustrative high-, middle-, and low-bandgap layers of the secondcrystalline semiconductor material 1500, is rotated by the flippingprocess, the layers may be in an optimal configuration to form aphotonic device.

Similarly, doping types in the layer or layers comprising the secondcrystalline semiconductor material 1500 may be chosen to take advantageof the bonding and flipping process. For example, later processing stepsmay raise the temperature of the device structure 1400 sufficiently tocause the material of substrate 310, e.g., Si, to diffuse into a firstdeposited layer or region in the second crystalline semiconductormaterial 1500. Because the material of substrate 310 may bean-typedopant in III-V materials such as GaAs and lnP, atoms of that materialthat diffuse into a first deposited p-type doped III-V layer maydeleteriously compensate the p-type dopants in that layer. Depositingn-type-doped III-V material on the substrate 310 first, however, mayinsulate other p-type doped III-V layers against diffusion from thesubstrate 310.

FIG. 16B depicts, in one embodiment, the removal of a portion of thesecond crystalline semiconductor material 1500 by, for example, anetching process. The dielectric layer 1404 may be relatively unaffectedby the etching process. In an alternative embodiment, a portion of thesecond crystalline semiconductor material 1500 and a portion of thedielectric layer 1404 may be removed simultaneously by, for example,CMP. The portion of the second crystalline semiconductor material 1500that is removed may contain at least a majority of the defects 1502,leaving the remaining portion of the second crystalline semiconductormaterial 1500 substantially defect-free. The exposed surfaces 1604 ofthe second crystalline semiconductor material 1500 may comprise (100)surfaces.

FIG. 16C illustrates the structure 1600, in one embodiment, after thedielectric layer 1404 has been removed. A portion of the dielectriclayer 1404 may have previously been removed by CMP, as illustrated inFIG. 16B, in which case the remaining portion of the dielectric layer1404 may be removed. The exposed second crystalline semiconductormaterial 1500 may comprise one or more ridges 1606. Each ridge 1606 mayhave a width w, corresponding to the width w of the openings or trenches1406. Adjacent ridges 1606 may be separated by a spacing s. The width wand the spacing s may each be less than or equal to a visible lightwavelength. In one embodiment, the exposed second crystallinesemiconductor material 1500 comprises a two-dimensional array of raisedfeatures. The width wand the spacing s of the ridges 1606 in a one- ortwo-dimensional array may be selected such that the ridges 1606 enhancethe light absorption or extraction efficiency of the structure 1600and/or improve the light beam quality. In one embodiment, the spacing sis approximately equal to the wavelength of the absorbed or emittedlight.

FIG. 16D illustrates another embodiment of the structure 1600. A metalcontact 1608, designed in accordance with standard methods, may beformed on the surface of the second crystalline semiconductor material1500. In one embodiment, the metal contact 1608 conforms to at least oneridge 1606. In an alternative embodiment, the oxide layer 1404 is notremoved before the metal contact 1608 is formed. In this embodiment, thetop surface of the structure 1600 may be planarized before the metalcontact 1608 is formed.

A photonic device 1510, such as a light-emitting diode or a photovoltaicdevice, formed in accordance with the method described in FIGS. 14-16,may exhibit several advantages. For example, an optical reflection layermay be formed, prior to bonding, on the handle wafer 1514 or on thesemiconductor structure 1400. The optical reflection layer may thereforebe disposed beneath the photonic device 1510, and may thereby enhancebacklight reflection and photon recycling. An LED photonic device 1510may exhibit increased light-extraction efficiency as a result of theplacement, width, and spacing of the ridges 1606 formed on the surfaceof the crystalline semiconductor material 1500. In addition, the ridges1606 may enhance the thermal cooling of the structure 1600, therebyimproving the thermal depletion of the structure.

Other embodiments incorporating the concepts disclosed herein may beused without departing from the spirit of the essential characteristicsof the invention or the scope thereof. The foregoing embodiments aretherefore to be considered in all respects as only illustrative ratherthan restrictive of the invention described herein. Therefore, it isintended that the scope of the invention be only limited by thefollowing claims.

What is claimed is:
 1. A semiconductor structure comprising: asubstrate; a metal layer extending over a top surface of the substrate;a photonic structure disposed above the metal layer, wherein thephotonic structure comprises a photonic device; and a first crystallinesemiconductor disposed above the photonic structure, wherein the firstcrystalline semiconductor comprises a plurality of ridges protrudingfrom a first surface of the first crystalline semiconductor, wherein thefirst surface of the first crystalline semiconductor is disposed abovethe photonic device, wherein the ridges of the plurality of ridges areparallel and extend in the same longitudinal direction, wherein a widthof one ridge of the plurality of ridges is less than or equal to avisible light wavelength, and wherein a spacing of the plurality ofridges is less than or equal to the visible light wavelength.
 2. Thesemiconductor structure of claim 1, further comprising a first metalcontact disposed on the first crystalline semiconductor.
 3. Thesemiconductor structure of claim 2, wherein the first metal contactextends on a sidewall surface and on a top surface of at least one ridgeof the plurality of ridges.
 4. The semiconductor structure of claim 1,further comprising a second metal contact disposed on a surface of thesubstrate opposite the first crystalline semiconductor.
 5. Thesemiconductor structure of claim 1, wherein top surfaces of theplurality of ridges are (100) surfaces.
 6. The semiconductor structureof claim 1, wherein top surfaces of the plurality of ridges arecoplanar.
 7. The semiconductor structure of claim 1, wherein thephotonic device comprises a light-emitting diode (LED), wherein thespacing of the plurality of ridges is about equal to a wavelength ofvisible light emitted by the LED.
 8. The semiconductor structure ofclaim 1, further comprising a second crystalline semiconductor disposedbetween the first crystalline semiconductor and the photonic structure,wherein the second crystalline semiconductor has a smaller bandgap thanthe first crystalline semiconductor.
 9. A device comprising: a firstmetal contact layer disposed on a first side of a substrate; areflective layer disposed on a second side of the substrate, the secondside being opposite the first side; a photonic layer disposed on thereflective layer, the photonic layer comprising at least one photonicsemiconductor device; a crystalline semiconductor material disposed onthe photonic layer, the crystalline semiconductor material comprising afirst crystalline semiconductor having a first bandgap and a secondcrystalline material having a second bandgap, wherein the firstcrystalline semiconductor is closer to the photonic layer than thesecond crystalline semiconductor, wherein the first bandgap is smallerthan the second bandgap, wherein the crystalline semiconductor materialcomprises first regions having a first thickness and second regionshaving a second thickness that is less than the first thickness, whereinthe bottom surfaces of the first regions and the bottom surfaces of thesecond regions are coplanar, wherein the first regions protrude abovethe second regions; and a second metal contact layer extending on thefirst regions and the second regions of the crystalline semiconductormaterial.
 10. The device of claim 9, wherein the reflective layer is alayer of aluminum.
 11. The device of claim 9, wherein adjacent firstregions are separated by a distance that is less than or equal to avisible light wavelength.
 12. The device of claim 9, wherein adjacentsecond regions are separated by a distance that is less than or equal toa visible light wavelength.
 13. The device of claim 9, wherein the atleast one photonic semiconductor device comprises a photovoltaic device.14. The device of claim 9, wherein the first regions extend over thephotonic layer in a first direction.
 15. The device of claim 9, whereinthe crystalline semiconductor material further comprises a thirdcrystalline semiconductor having a third bandgap, wherein the secondcrystalline semiconductor is closer to the photonic layer than the thirdcrystalline semiconductor, wherein the second bandgap is smaller thanthe third bandgap.
 16. The device of claim 9, wherein the first regionscomprise a two-dimensional array of raised features protruding above thesecond regions.
 17. A device comprising: a photonic structure disposedon a substrate, the photonic structure comprising one or moresemiconductor layers, the photonic structure having a top surface thatis planar; a crystalline semiconductor material on the top surface ofthe photonic structure; and a plurality of dielectric regions on thecrystalline semiconductor material, wherein the dielectric regions arelaterally separated, wherein the dielectric regions extend on thecrystalline semiconductor material in a first direction, whereinportions of the crystalline semiconductor material covered by thedielectric regions have a top surface that is a first distance above thephotonic structure, wherein portions of the crystalline semiconductormaterial that are free of the dielectric regions have a have a topsurface that is a second distance above the photonic structure that isgreater than the first distance; and a metal contact extending on thecrystalline semiconductor material in a second direction that isorthogonal to the first direction.
 18. The device of claim 17, whereinthe dielectric regions protrude above the crystalline semiconductormaterial.
 19. The device of claim 17, wherein top surfaces of thedielectric regions are coplanar with top surfaces of the crystallinesemiconductor material.
 20. The device of claim 17, wherein thecrystalline semiconductor material comprises a crystalline semiconductorhaving a first composition disposed on a crystalline semiconductorhaving a second composition that is different from the firstcomposition.